1. Field of the Invention
This invention relates to a method and system for processing sound signals in a vehicle multimedia system.
2. Related Art
Modern vehicle multimedia systems often comprise vehicle interior communication (intercom) systems, which can improve the communication between passengers, especially when high background noise levels are present. Particularly, it is important to provide means for improving the communication between passengers in the backseat and the front seat of the vehicle, since the direction of speech produced by a front passenger is opposite to the direction in which the passenger in the rear seat is located. To improve the communication, speech produced by a passenger is recorded with one or more microphones and reproduced by loudspeakers that are located in close proximity to the listening passengers. As a consequence, sound emitted by the loudspeakers may be detected by the microphones, leading to reverb/echo or feedback. The loudspeakers may also be used to reproduce audio signals from an audio source, such as a radio, a CD player, a navigation system and the like. Again, these audio signal components are detected by the microphone and are put out by the loudspeakers, again leading to reverb or feedback. Furthermore, the background noise level inside the vehicle constantly changes. In city traffic, the background noise level will be very different when the vehicle is standing, e.g., in front of a red traffic light, or when the vehicle is driving at moderate speed. At high vehicle speeds the change in background noise is even higher, for example when accelerating from 70 km/h to 130 km/h. To maintain intervehicle communication or to obtain audible sound from the multimedia system, the vehicle passengers have to continuously adjust the volume/amplification of the multimedia system. Adjustment of the volume can distract the driver and can lead to dangerous situations.
Furthermore, the vehicle passengers may want to be entertained during their journey. For this purpose, a karaoke system can be provided inside the vehicle. Such a karaoke system suffers from the same drawbacks as a vehicle intercom system, meaning that the reproduction of the voice from a singing passenger is prone to reverb and feedback. The volume of the sound output of the karaoke system also needs to be constantly adjusted by the passengers, depending on the background noise level inside the passenger compartment.
In the art, systems are known that use several microphones to record a voice signal produced by a speaking passenger. The detected sound signals are then processed for beamforming to obtain a directional sound signal in which sound originating from a direction other than a preferred direction is attenuated. The preferred direction is generally the direction in which the speaking passenger is located. That way sound originating from other directions like sound emitted from a loudspeaker is attenuated in the beam formed detected signal, whereby reverberation and feedback is reduced. Furthermore, background noise originating from directions other than the speaking passenger direction is attenuated by the beam former. Such beamforming systems are known from L. J. Griffiths, C. W. Jim: “An Alternative Approach to Linearly Constrained Adaptive Beamforming”, IEEE Transactions on Antennas and Propagation, Vol. AP-30, No. 1, pages 24-34, January 1982.
To cancel echoes and remove reverberation in recorded sound signals, adaptive filters are known in the art. If music is played back in the passenger compartment while the voice of the passenger is recorded, the components in the recorded signal resulting from a music playback may be attenuated using such an adaptive filter. Adaptive filters and algorithms for filter adaption are described in E. Häusler, G. Schmidt: “Acoustic Echo and Noise Control—A Practical Approach”, John Wiley & Sons, Hoboken, N.J., USA, 2004. The components from music playback are removed from the recorded signal by generating a compensation signal, which requires a signal from the music source as a reference signal. The compensation signal is then subtracted from the recorded signal. Such a system is described in the U.S. Pat. No. 6,895,095. The compensation of interfering components in the recorded sound signal is performed in the frequency domain, which requires extensive signal processing leading to delay. Delay is a major disadvantage in vehicle intercom systems or karaoke systems, since direct sound signals from the speaking/singing passenger and the sound from the intercom/karaoke system arrive at different times at the listening passengers leading to incomprehensibility. Furthermore, the system does not compensate for interfering signals for which no source signal is available. Furthermore, filter adaption in those systems is a problem for highly correlated sound signals such as voice signals. Present filtering techniques can therefore not provide a high quality sound signal that has been compensated for audio components and reverb/echo that has little time delay and can be utilized in vehicle intercom and karaoke systems.
Accordingly, a need exists for improved sound output of vehicle multimedia systems, and in particular, improved sound output to remove reverberation and feedback from detected sound signals while ensuring a good audibility of the sound output even at different background noise levels.